Towards Design –
Entity / Relationship Models
Richard Hopkins
rph@nesc.ac.uk
UML for Use Cases
NeSC, Edinburgh, Jan 5/6 2006
Contents
Goal
An appreciation of how UML class diagrams can be used
to elucidate technical detail
Contents
Internal vs External class diagrams
Details of class diagrams
Practical
UML for Use Cases, Jan 5/6 2006
Towards Design
2
Development Process
Capture
Requirements
to elucidate
requirements
– what
entities exist Activity Diag.
and how each
Class Diag.
behaves
Partially articulated
requirements
Use Case Diag.
Requirements
Sequence Diag.
Construct
Model of
Overall system
Class Diag.
…
Activity Diag.
Sequence Diag.
Structure
Behaviour
Requirements level Class Diagram – externally visible entities
Design level Class Diagram – model of external entities + internal
entities => OO design
UML for Use Cases, Jan 5/6 2006
Towards Design
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Internal vs External
User
Item
has out
Student
Staff Member
Copy
Is copy
of
Is part
of
contains
Journal
Book
Article
has out
Ordered print-off
User
Student
to
of
Loan
Staff for
print-off
order
Lendable
Journal
Copy
in
of
of Article
Book
Similarities – used external classes as starting point; differences, e.g.
A “lendable” entity is may be a more useful abstraction than
“Item” –
greater commonality
but need additional constraints for User-to-Loan
UML for Use Cases, Jan 5/6 2006
Towards Design
4
Internal vs External
User
Item
has out
Student
Staff Member
Copy
Is copy
of
Is part
of
contains
Journal
Book
Article
has out
Ordered print-off
User
Student
to
of
Loan
Staff for
print-off
order
Lendable
Journal
Copy
in
of
of Article
Book
Similarities – used external classes as starting point; differences, e.g.
Relationship entities – need information about the relationship –
when the loan is to be returned
In this case, 1:N, could put that info with the Copy
UML for Use Cases, Jan 5/6 2006
Towards Design
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Internal Entities
Introduce Modelling of required internal state
need to keep
track of fines –
what for
how much paid
Fine
Payment
Overdue
User
Student
to
Loan
Staff for
print-off
order
Damage
of
Loss
Lendable
Journal
in
of
of Article
UML for Use Cases, Jan 5/6 2006
Copy
Towards Design
Book
6
Contents
Goal
An appreciation of how UML class diagrams can be used
to elucidate techniocal detail
Contents
Internal vs External class diagrams
Details of class diagrams
Practical
UML for Use Cases, Jan 5/6 2006
Towards Design
7
Aggregation and Composition
Journal
Article
composition
Author
general aggregation
Special Associations
Aggregation – To model a collection of things
Journal has a collection of Articles
Article has a collection of Authors
Composition
A special case of aggregation – “ownership” / “component”
 An article is a component of a journal
Part can only belong to one whole
The part cannot exist without the whole
Role name - Not given explicitly – implicit in the end-point – “is
part of”
Multiplicity – by default (1) --- (0..*)
UML for Use Cases, Jan 5/6 2006
Towards Design
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Multiplicity
1..*
Fine
0..*
Payment
Overdue
1
1
Loan
User
Student
Staff
1
Damage
Loss
1
1
1
1
Lendable
1
print-off 0..*
order
Journal
Copy
Article
Book
1..*
Compactly communicates a lot of properties/assumptions – one picture of whole design
A payment can relate to several fines
An article can’t be in more than one Journal (issue)
….
Can be very useful as reality check on requirements with user
Can daunting notation for user – may be better to extract all assumptions made!
UML for Use Cases, Jan 5/6 2006
Towards Design
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Attributes
Copy
Book
Title : String
ShelfMark : Int
of
IsCopyOf : Book
Title : String
Attribute – Information associated with each instance of class
Each attribute will have a type (could be omitted)
Attributes are inherited – Title would be attribute of book, not
Copy
Show What’s important for understanding
Maybe not all that exists in implementation

Don’t show IsCopyof attribute because duplicates Of
association
Could put in virtual information

Title for copy even though only held for Book - ?
UML for Use Cases, Jan 5/6 2006
Towards Design
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Specialised Class vs Attribute
Vs.
Book
Book
hardBack: bool
HardBackBook
SoftBackBook
Use specialised class if a significant difference to user,
e.g. hardBack has a reBind operation, but softBack doesn’t
UML for Use Cases, Jan 5/6 2006
Towards Design
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Navigability
Is copy
of
of
Copy
Copy
Book
One-way
from copy to the
Book it is a copy of
Book
Two-way
Also From a Book to any of its copies
Arrow-head means the association can be followed in the
direction shown
In implementation view, navigability requires attributes to
hold the link(s) to associated object(s)
In VP-UML – default is no navigability signs –
either both un-specified or both navigable
UML for Use Cases, Jan 5/6 2006
Towards Design
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Complex Class Structures
Class Diagram
0..*
Book
by
0..*
1..*
Author
1..*
1
Division
1
by
0..*
Part
0..* contains
Chapter
The intended structure would be elucidated by giving some
examples !
Particularly if audience is not familiar with this particular view
on the possible structure of book
So illustrate the class diagram by one or more instance
diagrams
UML for Use Cases, Jan 5/6 2006
Towards Design
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Object Diagrams
Class Diagram
0..*
Book
by
0..*
1..*
Author
1..*
1
Division
1
by
An Instance
0..* contains
diagram
Does not show full
range of
0..*
Part
possibilities
Chapter
The lines are
“links” – instances
Object Diagram
B1: Book
Title: UML & WSDL
of associations
Can annotate links
A1: Author
Name: Richard
A2: Author
Name: Guy
C111: Chapter
Title: Classes
P11: Part
Title: UML
C112: Chapter
Title: Use Cases
UML for Use Cases, Jan 5/6 2006
P12: Part
Title: WSDL
with navigability
and role if useful
A2: Author
Name: David
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Questions ?
Can we express/derive full OO design from UML?
Yes
Additional features

operations

using UML-defined classes as attribute types

visibility

interfaces

….
?????
UML for Use Cases, Jan 5/6 2006
Towards Design
15
Contents
Goal
An appreciation of how UML class diagrams can be used
to elucidate techniocal detail
Contents
Internal vs External class diagrams
Details of class diagrams
Practical
UML for Use Cases, Jan 5/6 2006
Towards Design
16
Practical –
Entity/ Relationship Model
Clarifying technical detail – 1 hour
One group member plays role of software developer
draft a class diagram for the design the system you
have been working on
quickly!
could be two developers for 4-person group
Others play role of clients
with the developer – discuss and evolve the detail
The point is to explore the potential of these diagrams
as communication tool –
Not to come up with the ideal design
UML for Use Cases, Jan 5/6 2006
Towards Design
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